GHA: drop brew update from all jobs

After adding it a month ago (where missing) to fix a failure.

Removing this time to fix a different failure (on Linux), and also to
improve CI performance. Some install steps take over a minute, most of
that spent on `brew update`.

GH runner images also enabled extra taps which may contribute to further
delays, and seen to make it more fragile if GH itself struggles (taps
are hosted there.)

Refs:
https://github.com/curl/curl/actions/runs/27384213554/job/80927624171
https://github.com/curl/curl/actions/runs/27382368348/job/80921910973

Follow-up to db5d888673 #21608

Closes #21981

.github/workflows/checksrc.yml

@@ -74,7 +74,6 @@ jobs:
       - name: 'typos'
         timeout-minutes: 2
         run: |
-          /home/linuxbrew/.linuxbrew/bin/brew update
           /home/linuxbrew/.linuxbrew/bin/brew install typos-cli
           eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)"
           typos --version
@@ -150,9 +149,7 @@ jobs:
     steps:
       - name: 'install prereqs'
         timeout-minutes: 2
-        run: |
-          /home/linuxbrew/.linuxbrew/bin/brew update
-          /home/linuxbrew/.linuxbrew/bin/brew install actionlint shellcheck zizmor
+        run: /home/linuxbrew/.linuxbrew/bin/brew install actionlint shellcheck zizmor
 
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with: